The invention relates to a security system for surveilling the presence in defined zones such as, e.g. exit areas in shops or stores, of objects, for example commodities. The system comprises an antenna with at least one tuning capacitor in the monitored zone and receiver/transmitter devices functioning as tuned resonance circuits on the individual objects monitored, the antenna emitting at short intervals an electrical signal that makes the receiver/transmitter device of a monitored object, which is present in the monitored zone, oscillate, which oscillations in the intermissions between antenna transmissions can be received by the antenna--or by a separate antenna--and used to detect the presence of the monitored object in the monitored zone.
Such security systems are most typically used in shops and stores as a precaution against shoplifting, but may also be used in libraries to protect the books against theft. When an article has been sold or a book loaned, the receiver/transmitter device is removed from the article or book, respectively.
A problem inherent in such systems is that the antenna, which is a tuned circuit, unless special measures are taken, will continue oscillating for a short while into the intermission interval. It is then extremely difficult to distinguish between the residual signal of the antenna and a signal emitted by a monitored object.
In known systems of this kind attempts to solve the problem by shortcircuiting the antenna by the end of transmissions. Thereby, the energy stored in the antenna circuit is lost, but this requires a certain lapse of time, during which the antenna cannot receive possible reply signals from the receiver/transmitter device of a monitored object. During this lapse of time a very important part of the reply signal, which decreases exponentially, is lost, which has an unfortunate effect on the reliability of the monitoring process.